The present invention relates to sampling wood to identify wood species. More specifically the present invention provides a method of producing an ion mobility signature representing a wood sample and then comparing the signature with known signatures of wood species to determine the wood species of the sample.
When logs arrive at saw mills they are usually stacked, and in many cases it is difficult to tell the species of a specific log. It is known that different types of wood species have different commercial values and thus there is often an advantage for the saw mills to sort out different wood species in order to maximize the values of the more valuable woods. Today, distinguishing one species of wood from another is often done manually either in log form or when lumber is sorted, and this is not necessarily the most reliable or economical method. Once the species of the wood has been determined, there is generally a decision time of about 2 to 10 seconds while the wood moves along a conveyor to a position where the wood is sorted into a different species.
Ion mobility spectrometry (IMS) is recent technology which separates ionized compounds based on differences in their drift velocity through a gas under an applied electric field. This technique has the ability to produce a characteristic spectrum of the series of high molecular weight compounds in a matter of milliseconds. It is known that it can produce identifiable signatures for such items as drugs and explosives and is being developed for use by customs, airlines and police forces to detect such substances. Initial tests were carried out to determine if IMS could be used to identify different wood species. A report on these tests was published by A. H. Lawrence on Feb. 2, 1989 at the 75th annual meeting of the Technical Section of the Canadian Pulp and Paper Association entitled "Rapid Characterization of Wood Species by Ion Mobility Spectrometry". Some tests were carried out in the "positive mode" and some in the "negative mode". Positive mode is when polarity of the electric field is positive, i.e. positive ions present in the detection mode. Negative mode is when the polarity of the electric field is negative, i.e. negative ions present in the detection mode. The initial tests showed that some wood species could be identified one from the other provided the tests were conducted in both modes. However, there were a number of variable parameters that did not initially appear to be acceptable for use in the lumber industry. First of all sampling and analyzing by an IMS device took several seconds and this would hardly be feasible for fast moving conveyors used in saw mills. Secondly it seemed that only certain types of wood species could be identified and thirdly it was not clear how such a piece of equipment would work in a saw mill environment with saw dust, other types of particles as well as vapours from both machinery and wood are present.
Ion mobility spectrometers are known, and it is also known that specimens analyzed by such a spectrometer can produce different signatures, or plasmagrams as they are sometimes referred to, which are affected by many different variables, e.g. temperature, barometric pressure, humidity, etc. Furthermore, when one analyzes a specimen of wood, the wood may be dry or moist. Heartwood and sapwood from the same wood species have different ion mobility signatures, and there are other effects such as extraneous noises, radio signals, vibrations etc. that may effect the signature of a trace sample.